Wash arrangement for screen centrifuges



8, 1969 P. NYROF' ET AL 3,424,312

WASH ARRANGEMENT FOR SCREEN CENTRIFUGES Filed Nov. 25, 1966 Sheet of 5 INVENTORS. PER NYROP J MES c. ELSKEN AGENT Jan. 28, 1969 p, NYRO ET AL WASH ARRANGEMENT FOR SCREEN CENTRIFUGES Filed Nov. 25, 1966 SCREEN WASH LIQUID PRIOR ART DISCHARGE QRFICE "WASH LlQUID v DISCHARGE ORFICE WASH SPRAY INUNDATION AREA Sheet 5 of3 I NVENTORS.

PER' NYROR JAMES c. ELSKEN United States Patent 3,424,312 WASH ARRANGEMENT FOR SCREEN CENTRIFUGES Per Nyrop, Norwalk, Conn., and James C. Elsken, Elmhurst, Ill., assignors to Dorr-Oliver Incorporated, Stamford, Conn., a corporation of Delaware Filed Nov. 25, 1966, Ser. No. 597,153 U.S. Cl. 210-374 Int. Cl. B0ld 33/06 3 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to screening centrifuges of the type having a rotor comprising a separating screen and a cage attached to the screen, and a conveyor coordinated with the rotor to move solids along the screen. In such devices the feed material is introduced at one end of the rotor. Rapid rotation of the rotor creates a centrifugal force which throws the feed material radially outward against the screen. As is well known, the continued rotation of the rotor causes a separation of the feed material, such separation being a function of the relative particle sizes of the components that make up the feed material; the solids being trapped and retained on the screen, the liquid or mother liquor being centrifugally forced through the screen.

The conveyor of the centrifuge has helical vanes and is positioned within the rotor concentric therewith, with a predetermined, clearance therebetween. The helical conveyor is also rotated but at a slightly different speed than th rotor. In this manner the helical vanes effect a traverse of the screen, from the feed end to the discharge end thus controlling the axial movement of the solids along the screen.

After the initial separation of liquids and solids, the solids retained on the screen are moved axially, under the control of the conveyor, to a wash zone. A washing liquid is applied and any impurities or remaining mother liquor is removed.

Many problems have arisen in this area but the most pressing concern is that a significant amount of the wash liquid never comes in contact with the solids on the screen. Two rationals have been presented to explain the relative inefliciency of the washing zone in prior art devices. The first is that the spray characteristic of the wash liquid, once discharged radially in the direction of the retained solids, cannot be sutficiently controlled so that it can be directed to a particular area on the screen. As can be seen from FIGURE 4 the combination of forces acting on the wash liquid, once it is discharged, spread the wash liquid over a greater area than the crosssectional area of the discharge port. The second explanation is that the retained solids tend to congregate along the leading face of the blade of the helical conveyor. The result, as shown in FIGURES 3 and 4, is a sort of thick rolling rope which is slowly rotated as the retained solids are moved axially along the screen by the conveyor blades. The solids rope is too thick to be fully penetrated by the wash liquid so that only the outer crystals are mixed with the wash liquid. Thus when the washing zone is viewed as a whole it can be seen that a large part of the wash liquid never comes in contact with the solids, the axial distance covered by the solids rope being only a small portion of the spray area inundated by the wash liquid; and, the wash liquid which does come in contact with the solids does not sufficiently penetrate the solids rope so as to mix with and scrub those crystals within the solids rope or those crystals immediately adjacent to the blade and to the screen.

Applicants addressed themselves to these problems by completely reorienting the screen and cage area directly opposite the discharge ports of the Washing apparatus. Their solution was to abruptly change the longitudinal direction of the screen by interposing a band in the screen at a greater angle of conicity than at least the upper portion of the screen. The angle of conicity being determined by the angle the sides of the rotor make at the feed end of the centrifuge. In this manner as the solids rope reaches the washing zone the increase in inclination of its longitudinal path of travel causes the solids rope to disintegrate and the solids to be accelerated and slide along the band in a thin film-like flow. Preferably, the helical vanes of the conveyor are spaced away from the inclined band to permit the solids to freely slide along the band without interference from the conveyor.

This phenomenon can be illustratively shown by placing a pile of granular material on a horizontal surface. If the pile is slowly pushed off the horizontal surface onto an inclined surface, the grains will accelerate, change their configuration by spreading themselves out, and slide down the incline in a thin layer. The first grains to reach the acceleration incline are moving faster than the rest of the grains still in the pile. Since the acceleration of the grain is constant, the grains will increase their velocity along the entire length of the incline. With the speed of the grains thus constantly increasing the first grains to reach the acceleration incline will be well along the incline before other grains in the pile start to flow and accelerate. The pile is thus thinned out along the entire length of the incline.

Incorporating this into the wash zone of a screening centrifuge the result is a relatively thin film-like formation of solids passing over the inclined band. If the longitudinal dimension of the band covers the full spray area inundated by the wash liquid a more thorough mixing of the wash liquid with all the solids is possible and all the solids will be effectively and efliciently scrubbed by the Wash liquid.

Applicants have also found by making the inclined band imperforate it not only enables all of the wash liquid to mix with all of the retained solids but because the wash liquid cannot pass through the imperforate band they have in effect, retained the wash liquid in contact with the solids as long as the solids remain in the wash zone. Thus the actual wash period is increased without increasing the amount of wash liquid used and without increasing the size of the wash zone.

The discontinuance of the screening area by the interposition of an imperforate band in the washing zone has provided a further advantage in certain industrial applications. In some processes the mother liquor and the wash liquid are different fluids and it may be undesirable to contaminate one with the other. In such situations because the mother liquor is separated from the solids in one distinct and separate area of the screen, and the solids are Washed in another and separate area of the screen the integrity of the mother liquor and wash liquor can be maintained.

A still further advantage which has evolved by the incorporation of applicants novel washing zone is a reduction in maintenance costs by dividing the screen into two sections. Studies have shown that the screen has the greatest wear rate in the area where the solids and mother liquor are separated. Therefore instead of replacing the entire screen, when only a portion of it is worn, only the worn section need be replaced.

Accordingly it is the primary object of the present invention to change the configuration of the solids retained on a centrifuge screen as the solids pass into the centrifuge wash zone to permit more thorough washing of the solids.

It is a further object of the present invention to provide an imperforate band interposed between two centrifuge screen sections at a greater angle of conicity than the screen sections.

It is another object of the present invention to provide means to change the flow characteristics of the retained solids on a centrifuge screen when they enter the wash zone.

Yet another object of the present invention is to provide means for maintaining the integrity of the mother liquor and wash liquid and to prevent their intermixing.

The subject matter which applicant regards as his invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. The invention, however, as to its organization and method of operation together with further objects and advantages thereof will best be understood by reference to the following description taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a schematic section of a screening centrifuge in which the washing arrangement of the present invention has been incorporated;

FIGURE 2 is a vertical section showing the structure of the rotor, drive, housing and washing arrangement of the centrifuge of FIGURE 1;

FIGURE 3 is a view partly in perspective and partly in section of the helix, screen and cage which illustrates the rope-like configuration which the solids retained on the screen assume as they are moved axially along the screen under the control of the helical conveyor and, the layer-like configuration in the wash zone which the solids assume as a result of a relatively inclined annular imperforate band; and

FIGURE 4 is a vertical section of a part of a prior art helical conveyor showing the washing discharges on the helical conveyor and the screens.

Referring to the drawings, a screening centrifuge is shown to illustrate an exemplary application of the present invention. However, it should be understood that the present invention is not limited in its application to the particular centrifugal apparatus disclosed.

As shown in schematic form in FIGURE 1, screening centrifuge 10 has an inlet opening 12 for feeding material to be processed to a rotor 14 as indicated by arrows 16. The feed material passes into an annular chamber 18 in the rotor between a rapidly rotating perforated cage 20, which carries a separating screen 21 (FIGURE 2) concentric therewith, and a rotating conveyor or helix 22. Rapid rotation of cage 20 forces the feed material radially outward against the screen. Continued rotation causes a separation of the feed material with the screen retaining one portion of the feed, e.g., the solids, while the other portion, e.g. liquid or mother liquor is centrifugally forced through the screen as indicated by arrows 24. The mother liquor is collected in annular chamber 26 in the centrifuge housing 28 and from there discharged through conduit 30 for further processing or reuse. The solids retained on the separating screen 21 are conveyed axially toward the discharge end of the screen under the control of the helical blades or vanes 31 on conveyor 22. The conveyor rotates in the same direction as the cage 20 but at a slightly dilferent speed, thus effecting an apparent traverse of the screen from the feed end to the discharge end.

The retained solidsare then subjected to one or more wash zones, depending upon the solids and the desired degree of purity, to remove any remaining mother liquor and/or impurities. Wash liquid scrubs the retained solids and then passes through the separating screen 21 into annular chambers 25 and 27 for subsequent discharge through openings 29. The solids are finally discharged from rotor 14 through openings 32 as indicated by arrows 34 into chamber 36. The solids are collected and retained in chamber 36 and from there discharged through out- I let 38 as indicated by arrow 40.

As shown generally in FIGURE 1 and more specifically in FIGURE 2, a drive and housing assembly 42 includes a fixed, double-cone-shaped, support housing 44 having annular bearing assemblies 46 and 48. A concentric inner shaft assembly 52 connected to the upper end of helix 22 is mounted by bearing assemblies 54 and 56 to rotate coaxially within outer shaft assembly 50.

The lower ends of shaft assemblies 50, 52 are driven at relatively differential speeds by a suitable drive ar rangement such as transmission 58 powered by pulley 60, driven by a belt or belts 62 which extends from a power source such as a motor (not shown) into housing 28 through a suitable belt opening 64.

Rotor '14 includes an annular ring 70 suitably secured to the upper open end of cage 20 which forms feed inlet opening 12. Cage 20 has a plurality of openings 76 therein for passage of effluent to the fluid chambers which surround the cage. The cage is secured by screws 78 to a ring 80 containing the solids aperture 32. The ring 80 is in turn secured by screws 82 to a cage hub 84 and the hub is non-rotatably mounted on hollow shaft 86 of outer shaft assembly 50 by any suitable means.

Helix 22 includes a lower conical portion 90 secured by a key 92 to rotate with a shaft 94 of inner shaft assembly 52. Helix 22 further includes an upper conical portion 96 which is connected to lower portion 90 by any suitable means such as cap screws 98 and pins 100.

Outer shaft assembly 50 further includes a bearing spacer 102 between bearings 46 and 48, retaining assembly 104 for bearing 56, and a lock nut assembly 106 to retain bearings 48 as well as bearing spacer 102 and bearings 46 in place. Inner shaft assembly 52 is provided with an oil sealing arrangement 108 between cage hub 84 and inner shaft 94 and a lock nut assembly 110 to retain bearing 56 on shaft 94. Support housing 44 has a seal assembly 112 which provides a seal between the stationary housing and the rotating hub 84 and a retainer 114 to maintain bearings 46 in position.

Referring now to FIGURES 2 and 3 wherein applicants novel washing arrangement is shown, it should again be understood that the incorporation of applicants washing arrangement in the centrifuge shown in FIG- URES 1-3 is by way of example only; applicants device is not intended in any way to be limited to this particular centrifuge. It should also be understood that while only one such washing arrangement is shown in this particular centrifuge this is for illustrative purposes only. It should be within the purview of those skilled in the art to incorporate applicants washing arrangement for all the washing zones in a particular centrifuge or for any selected washing zones.

The retained solids after separation from the mother liquor, wend their way axially under the control of helical blades 31 along the screen 21 until they enter washing zone 120. An imperforate band 124 having a leading edge 122 and a following edge 123 is inclined relative to the upper portion of the screen and cage at a greater angle of conicity than the screen (the angle of conicity being that angle which the sides of the rotor make at the feed end of the screening centrifuge). As explained in the introduction to the specification, the abrupt increase in the angle of conicity in the longitudinal path of the retained solids is analogous to shifting a pile of granular material from a horizontal surface to an inclined surface.

The incline will accelerate the individual grains as they are moved from the horizontal surface to the inclined surface and continue the acceleration, thereby increasing the velocity of the grains, as they continue along the inclined surface. The result is that the pile will become thinned out, i.e., the first grains to reach the acceleration incline will be well along the incline before other grains in the pile reach the incline and begin to accelerate and increase their velocity. Thus when the rope-like formation of solids approaches the leading edge 122 of imperforate band 124, the retained solids accelerate, change their configuration and assume a relatively thin film-like flow. Helix 22 is provided with two series of passages 126', 128, 130 and 132, 134, 136, 138 to supply a wash fluid or fluids from concentric wash fluid feed pipes 140 and 142 to the solids retained on screen 21. The wash fluid from passage 130 discharges out through a series of nozzles 144 to inundate the imperforate band from the leading edge 122 to the following edge 123 thereby thoroughly scrubbing all the retained solids. Since the wash liquid cannot discharge from the retained solids in the wash zone, i.e., the wash liquid cannot pass through imperforate band 124, the wash liquid will remain with the solids as long as they remain in the wash zone.

As can be seen in FIGURES 2 and 3 the rotor 14 has a stepped configuration with the upper and lower portions being joined by the imperforate washing zone 120. Upper rotor portion 150 includes separating screen 152 and cage 154. Washing zone 120 is defined by solid band 124 which may be integral with the rotor or may be separate and joined to the rotor by any suitable means such as cap screws 156. Lower rotor portion 158 includes separating screen 159 and cage 160.

Because the rotor 14 has been redesigned to incorporate applicants washing apparatus it has also become necessary to redesign the helical conveyor 22 so that the retained solids may be transported along lower screen portion 158. As shown by the engineering-drawing representation of helical blades in FIGURE 2 and by the perspective drawing of FIGURE 3 the blades 31 follow the contour of the rotor 14, i.e., the upper portion of the helical conveyor has its blades immediately adacent to, and at the same angle of conicity as upper rotor portion 150 and the lower portion of the helical conveyor has its blades immediately adjacent to, and at the same angle of conicity as, lower rotor portion 158. The blade is recessed in the immediate vicinity of inclined band 124. This enables the rope'like configuration of solids to disintegrate and slide down the band in a thin film-like flow unhindered by any interference from a blade 31 of the conveyor. When the washed solids leave the washing zone, their axial movement is once again under the control of helical blades 31, and this continues until the solids are discharged out of the rotor through exits 32.

It can therefore be seen that applicants have invented a novel and inventive washing arrangement for centrifuges which arrangement insures a maximum amount of washing effort from a minimum amount of wash liquid, with all of the retained solids being thoroughly inundated and scrubbed by the wash liquid. Applicants novel arrangement also allows for an improved separate collection of mother liquor and wash liquid thus retaining the integrity of each applicants arrangement further improves upon the prior art by providing two distinct separating screens, thus when only one becomes worn, only it has to be replaced.

As this invention may be embodied in several forms without departing from the spirit or essential character thereof, the present embodiment is illustrative and not restrictive. The scope of the invention is defined by the appended claims rather than by the description preceding them and all embodiments which fall within the meaning and range of equivalency of the claims are therefore intended to be embraced by those claims.

We claim:

1. A screening centrifuge comprising:

(a) an annular screening means to fractionate feed material being treated in said centrifuge into a fraction retained on said screening means and a fraction centrifugally forced through said screening means, said screening means having means intermediate its ends to accelerate the longitudinal velocity of the solids retained on said screening means;

(b) conveyor means, coaxial with and concentric to said screening means, having a feed end and a discharge end and at least one helical vane, said vane being inclined relative to the axis of rotation of said conveyor means and following a path of substantially constant angle of inclination from the feed end of said conveyor means to the discharge end of said conveyor means to substantially continuously control the movement of the fraction retained on said screening means as it moves axially along said screening means;

(0) at least one wash means in said conveyor means to discharge a wash liquid against the fraction retained on said screening means, said wash means having at least one discharge means to direct the outward flow of the wash liquid, said discharge means being positioned directly opposite the accelerating means on said screening means; said vane of said helical conveyor releasing its control on the movement of the fraction retained on said screening means as the fraction approaches the zone defined by the accelerating means on said screening means and regaining control of the movement as the retained fraction leaves the zone defined by said accelerating means.

2. A screening centrifuge as defined in claim 1 wherein said accelerating means is an annular section of said screening means having a greater angle of conicity than at least one portion of said screening means.

3. A screening centrifuge as defined in claim 2 wherein said annular section of said screening means has means thereon to prevent passage of liquid therethrough.

References Cited UNITED STATES PATENTS 568,821 10/1896 Waring et a1 210374 X 2,040,822 5/ 1936 Brewer 21 0-3 X 2,283,457 5/1942 Pecker 210-374 FOREIGN PATENTS 859,429 12/1952 Germany.

REUBEN FRIEDMAN, Primary Examiner.

I. L. DE CESARE, Assistant Examiner.

US. Cl. X.R. 210-378, 380 

